Variable resistor



March 8, 1960 K. w. JARvls VARIABLE RESISTOR 2 Sheets-Sheet l Filed Aug.27, 1956 FHEu FII

March 8, 1960 l K. w. JARvls 2,927,367

VARIABLE RESISTOR Filed Aug. 27, 1956 2 Sheets-Sheet 2 mp .am l

'United States Patent y 2,927,367 vARrABLE REsrsroR Kenneth W. Jarvis,Winnetka, Ill.

Application August 27, 1956, Serial No. 606,408

3 Claims. (Cl. 291`55.68)

The present invention relates generally to variable resistors, and morespecifically to variable resistors in which a given displacement of themovable contact on the resistance element produces a precise change inresistance.

The invention also relates to methods of manufacturing variableresistors.

Most variable resistors for use in devices requiring relatively smallcurrents, such as electronic devices, `are v constructed with resistancecards which have a plurality of turns of resistance wire woundtransversely around a strip of electrical insulating material. Aslidable electrical contact is in physical contact with a Anarrow -bandof exposed turns which extends generally normal to the direction of the`turns, so that movement ofthe Aelectrical contact along this band formselectrical contact with differentportions of the resistance card. If thechange in resistance is to be linear with displacement of the electricalcontact, each of the turns must be separated by an identical distance,and the total resistance around each turn must be identical. Neither ofthese requirements can befreadily accomplished except by expensiveprecision equipment. To accomplishthe first requirement, it is necessaryto Wind the turns of resistance wire onto the electrically insulatingstrip with great accuracy and to secure them in place before any changein their position can occur. The second requirement is hampered by thefact that resistance wire has a non-uniform incremental resistance alongits length, largely due tovmanufacturing dilhculties.-

kIt is one of the objects of the present invention to provide a variableelectrical resistor which exhibits the desired relationship betweendisplacement ofthe electrical p contact and resistance with precisiongreater than that obtainable by ordinary production methods. p

It is also'an object of the present invention toprovide a method ofmanufacturing precision variable re sistors which corrects theerrorinthe contact displacement resistance relationship ofthe resistor afterthe resist- `ance card has been formed. f i

The dilhculty in constructing a variable resistor having y a. preciseresistance to displacement of the electrical contact relationship ismagnified if the resistance to displacement relationship is to be otherthan linear, for example, exponential. At present, variable resistors ofthis type are wound with different spacings between adjacent turns of aresistance wire on the resistance card, or by having a resistance cardwound about an electrically insulating strip of increasing width.Further, the precision of the relationship between resistance anddisplacement of the electrical contact for such a resistor is'limited bythe samev Patented Mar. s, 1960 "ice g These andother objects of thepresent invention will be readily apparent to the man skilled in the artfrom a further reading'y of the present disclosure, ,particularly whenkviewed in the light of the drawings, in which:

Figurel is a sectional view of a resistor constructed according to theteachings of the present invention, the section being taken along theline 1-1 ofFigure 2;

Figure 2 is a sectional view of the resistor taken along the line 2 2 ofFigurel;

Figure 3 is an elevational view of the opposite side of the contact discfrom that illustrated in Figure l;

Figure 4 is a sectional view taken along line 4 4 of Figure 2;

Figure 5 illustrates one of the steps in the manufacturing process andshows an elevational view of the re-l sistance card; i

Figure 6 is a sectional view through the resistance card in another stepin the manufacturing process;

Figure 7 is a sectional view illustrating still another manufacturingstep; and

Figure 8 is an enlarged fragmentary sectional view taken along the line8--8 of Figure 6.

The variable resistor illustrated in the figures has a hollowcylindrical casing 10 with a closed end 12 and an open end14. A cover 16is secured to the open end 14 of the casing 10 by press-tting the cover16 in a groove 18 about theinner perimeter of the open end 14 of thecasing 10.- The closed endl 12 of the casing llis. provided with acentral aperture 2,0.and has a sleeve 21 which extends toward the openend 14 ofthe casing 10. A bushing 22 is secured to the casing 10 anddisposed within the aperture 2li. The bushing 22 is provided with asleeve 24 which extends into the sleeve 21 'coaxially therewith. A shaft26 is journaled within the bushing 22 and sleeve 24. The shaft isprovided wtih a groove 2.7 adjacent Ato the bushing 22 and a C-washer 28is d isposed in the groove 27. The cover 1.6 is also provided with anaperture 29, the shaft 26 extending throughthe aperture, and a secondC-Washer 3% is disposed in a groove 31 in the shaft adjacent to thecover 16, thereby preventing translation of the shaft 26 within thecasing and limiting its motion to rotation. y p I The interior of thecasing 10 is lined with an annular strip 32 of electrically insulatingmaterial whichis s ef cured to the interior surface of the casing 10,rfor example,l by a layer 34' ofv cement.

posed in contact with the strip 32 of electrically insulat ing 10 in theinterior thereof. A mass of potting compound 37 is also disposed withinthe casing 10 and extends to a level adjacent to the edge of resistancard 36.

The resistance card 36 is provided witha strip 38'of electricallyinsulating material which is disposed in an annular form Within thecasing 10. The strip 38 supports a plurality of turns 40 of resistancewire 42. The resistance wire 42 is provided with a coating 44 ofelectrically insulating material, such as enamel for example. Af layer46 of electrically insulating cement is disposed about the strip 38 andwindings 40 thereabout, except fortwo bands 4S and 49 which extendlongitudinallyv along the lresistance card.l The first band 48 isdisposed on the edge of K the resistance card confronting the cover 16ofthe variable resistonrand the turns 40 along this band 48 are alsofree of the electricallyinsulatinglcoatingfii. Thev second band 49 isdisposed centrally of the side of the' rel sistance card confronting theshaft 26 and extends longitudinally about the resistance card. This band4.9v also is a band in which the insulating coating 44 on the turns' 40is removed, so thatelectrical contact may be made directly with the wire42.

A resistance card 3.6, whicirwill be later described in greaterdetaihisdisg electrically conducting material upon the exposed portions of theturns 40 in the second strip 49 is to short out this portion of eachturn, and the thickness of the coating should be suliicient for thispurpose but as thin as practical otherwise, since too great a thicknessof coating tends to bridge between adjacent turns. A layer 52 ofelectrically insulating material is disposed over the second band 49 andover the coated portion of the turns in the second band 49.

A disc 5S of electrically insulating material is secured to the shaft 26at its center adjacent to the end of the sleeve 24, and a pair ofelectrically conducting contacts 60 are mounted to the disc 58 Vadjacentto its periphery. Each of the contacts 60 has a wire shaped contactmember 62 which is disposed in slidable contact with the exposed portionof the turns 4G on the edge 48 of the resistance card 36. Each of thecontacts 6i) has a supporting bar 64 which is secured to theelectrically insulating disc 58 in an indentation 65 generally parallelto and adjacent to the periphery of the disc 58, for example by a pairof screws 66 disposed at the end of the support bars 64. The support bar64 of each contact 6i) is provided with an indentation 6a confrontingthe resistance card 36, and a pair of aligned screws 70 and 72 generallyparallel to the axis of the support bar 64 are threadably engaged withinsupport bar 6d and extend through the support bar A64 into theindentation 68 to confront each other. A pin 82 extends between theconfronting ends of the screws 72 and 70 ywithin the recess.

The contact member 62 of each contact 60 is in the form of ka ridgedelectrically conducting wire which is secured to the ends of the pin 82,as by soldering for example, and makes electrical contact with the pin82. Each contact member 62 has three adjacent portions 84, 86 and 88.The central portion 86 is disposed normal to the pin 82 betweenthe endportions 84 and 86 of the contact member 62 on the side remote from theelectrically insulating disc 58. A helical spring 9i) is also disposedabout each pin 82 between the ends thereof, and one end 92 of the spring9G extends outwardly from the helix `and abuts vthe support bar 64 toplace a spring bias upon the contact member 64 inthe direction of the`resistance vcard `36. In -this vmanner the central portion 8 6 .of each.contact member 62 is maintained in abutment with theexposed portion ofthe turns 40 on edge 48 ofthe resistance lcard 36.

The electrical linsulating disc 58 also supports a Ypair of concentricelectrically conducting rings 94 and 95 which are coaxially disposedabout the shaft 26 on the side lof thedisc 58`confronting the closed end12 of the casing 10. The ring '94 is electrically connected to one ofthe contactmembers 6) and the ring.9 is connected to the othercontact-member 60 by wires-96a yand 96h, respectively.

vThe contact rings 94 and-95 are leach in slidable contact with-a -pairofcontact assemblies 97 and 99 which have a pair kfot electricallyconducting contacts 93 .and 11m-of a take-oit assembly 102. The take-olf`assembly 102 has an electrically insulating block 104 which is mountedto theend 12 of the casing 19 confronting the contact rings 94 and`S35i, asillustrated in EigureZ. `A pair-of parallel-slotsl 196 and i6?generally parallel to and-confronting the contact .rings 94 'aridp95 aredisposed in the surface of the block lil-fi. Each of the contacts :98andr100 are mounted attheends .of arms108 and 110, respectitely, whichare constructed of electrically conducting material. Ea-ch pair @farms108 vand 110 is pivoted at a point between their ends on a pin 112 whichi extends into the block 104 of insulating material normal to the armsldd and 11) and through one of the slots 166 or 167 in the block` Ahelical spring 114 is disposed about the pin between each pair of armsE68 and 11i?. One end of each spring lid is attached to the end of armitl@ opposite to the contact 98, and the i Yother end of each springlll/i is attached to the adjacent arm lli opposite the contact led,thereby spring-biasing these ends of the arms tlt and il@ toward theslot 166 or HE7, or in other words, spring-biasing the contacts 98 andlili? outwardly from the slot 166 or 167.

The arms i and 11d of each Contact assembly 97 and 99 are electricallyinterconnected by the pins M2. Also, one of the arms ltl of contactassembly 97 is connected to a terminal H6, and the arm of Contactassembly 99 is connected to a terminal llll. Electrical contact to theother arm of each pair of arms is made through the pins i12. Electricalconnections may be made to these terminals 116 and E8 and to one or morepoints on the resistance card, in the conventional manner.

The inventor has found that a preferred method of manufacturing theprecision variable resistor described in Figures l through 4 is asfollows: The turns di) of enamel coated resistance wire are wound ontothe strip' 38 in the conventional manner, generally with the aid ot acoil winding machine to assure even spacing between turns. The turns 40are wound generally parallel to each other and equally spaced from cachother. The layer 2id of electrically insulating cement is then appliedentirely over the strip 33 and turns fit?. The two bands 48 and 4,9arethen cleared of the layer of insulating cement 46,. and enamelcoating 44-, the one band. itl being along one edge ofthe resistancecard, and the econdrband 43 being parallel with this edge centrally ofone of the sides of thev card, This operation may be done withy a wirewheel or a scraper. v

An electrically conducting strip, such as a right angled trough,designated 13@ in Figure 7, is then provided with a pad of electricallyconducting wool 132 in its trough, and the edge 4S or the resistancecard is secured within this trough. The wool 132 may be steel wool. Theresistance card 36 is then lowered into a container 134 of an'4electrically conducting solution 136. The inventor has found ythat goldcyanide is a particularly satisfactory solution for gold coating theexposed portions of the turns 40 upon the resistance card 36. Anelectrical potential is applied from a battery 133 between an electrode140 in the bath of gold cyanide and the electrically conducting strip13d, thus causing an electrical current to ow through the bath anddeposit a gold coating 50 upon the portion of the turns 40 of theresistance wire l2 in the band 4% exposedto the solution. Care is takenthat the coating 50 deposited jon these turns 46 does not bridge betweenturns.l l

The resistance to displacement characteristic of the variable resistormay be adjusted either prior to bending the resistance card 36 or afterplacing the resistance card 36 in the casing 14B. It is preferable,however, to bend the resistance card 36 and secure it in the casing 16prior to adjusting this characteristic, since the adjustment will thencompensate for concentricity errors and other errors which maybeintroduced in the assembly process.

After the coating Sil has been placed on the wire 42 and the resistancecard 36 placed in the casing 10, the resistance is measured between oneofthe ends of the resistance wire, designated ldl, and the exposedportion of each turn of resistance wireV 42 in the band 48 by connectingone probe M2 o1c an ohmmeter E46 to the cndvldl ofthe resistance wireand the other probe 14d toeach individual turn in the band rlfhe totalresistanceV from the end lil to each turn is then adjusted to a desiredvalue by scraping enough of the coating 50 of gold from the wire 42 inthe band 49 to adjust the resistance of each turn to the desired value.In this way, a constellt resistance increment is added by addingadditional turns into a circuit and the resistance to displacement ofthe movable contact 62 is a linear function.

For some applications, variable resistors need not be completelyaccurate throughout their range. Variable resistors for suchapplications can be adjusted to the desired relation between resistanceand displacement of the movable contact in steps. The probe 144 maycontact only the end turn of each of a plurality of groups of turns, asindicated by the dotted lines in Figure 5, and the coating 50 withineach group adjusted', as required, vthe amount of coating 50 beingremoved on each turn 40 of each group uniformly. When constructed inthis manner, the relation between resistance and contact displacement isaccurate only at the end of each group of turns. There may be any numberof turns in a group, all groups having the same number of turns, forexample 20 to 25 turns.

The electrically conducting coating 50 placed upon each of the turns 40in the band 49 may be of many different electrically conductingmaterials, for example copper, silver or aluminum. However, a very finegrain coating is desirable since it is easier to control the thicknessof a tine grain coating and prevent bridgingbetween turns. Corrosionresistance of the coating is also important for some applications. Forthese reasons, the inventor has found it preferable to use gold ratherthan some other material for this coating 50. In a particularconstruction, the width of the band 49 was 1A inch wide, while the widthof the electrically insulating strip 38 isapproxmately 1% of an inch.

After the resistance to contact displacement characteristic of theresistance card has been adjusted, the coating 52 of electricallyinsulating material is placed over the band 49 and the layer 46, forexample by brushing it l on. The variable resistor may then be furtherassembled in a conventional manner.

An exponential electrical characteristic may be built into a variableresistor of the type here described by constructing the strip 49 in anexponential form, as indicated by the dotted line in Figure 5 anddesignated 49a. It is clear that each turn 40 of resistance wire 42 hasan increasing total resistance considering the turns from right to left.As a result, the sliding contact 62 adds a larger amount of resistanceto the total resistance between the left end of the resistance card 36and the sliding contact 62 for each turn moving in this direction. Also,an exact exponential relation between contact displacement andresistance may be obtained by adjusting the resistance of eachindividual turn 40 by scraping a portion of the coating 50 from the wire42, as described above for a linear relationship. t v

The present invention may also be employed using fixed resistance incombination with a resistance card, as illustrated'in dotted lines inFigure 4. The resistance card 36 is provided with a plurality of taps150, here equally spaced and beneath the surface of the potting compound37. A fixed resistor 152 is connected between each pair of adjacent taps1750, the resistors 152 having equal resistance for a linear voltage toresistance characteristics and an equal number of turns between taps 150on the resistance card 36. v

The eifective resistance, R, between each pair of taps is the resistanceof the turns on the resistance card 36 between these taps 150.v

The voltage-resistance characteristic of the resistor is adjusted byremoving a portion of the plating 50 from the band 49, as describedabove. This construction permits larger currents through the resistor,than with the resistance card alone, and also the use of precision fixedresistors.

It is also clear that virtually any displacement toresistancecharacteristic may be obtained by shaping the strip 49 to titthe requirements and additionally adjusting the resistance of yeachturn. Further, the layer 46 may be disposed only on one side of theinsulating strip 38, in an extreme case, or omitted entirely, before thecoating process places the coating 50 on the wire 42, thus providing amuch greater range of adjustment to produce the desired resistance todisplacement of the slidable electrical contact.

The man skilled in the art will readily devise many modifications andadditional applications for the invention beyond those specificallydisclosed herein. It is therefore intended that the scope of theinvention be not limited bythe foregoing disclosure, but rather only bythe appended claims.

I claim as my invention:

1. The method of manufacturing a resistor comprising the steps ofwinding a plurality of turns of resistance wire around a strip ofelectrically insulating material, coating the strip and turns withelectrical insulating material, removing a band of insulating materialfrom the strip and turns along one edge of the strip, removing a secondband of insulating material from the side of said strip and turns,placing an electrically conducting member into contact with the band ofturns on the edge of the strip, immersing the second exposed band ofturns into a plating bath, applying an electrical potential between theelectrically conducting member and an electrode disposed in the platingbath to deposit an electrically conducting coating on the turns,reducing the length of said coating on individual turns to adjust theresistance of said turns, placing a second layer of electricalinsulating material on said strip and turns.

2. The method of manufacturing a variable resistor comprising the stepsof claim 1, placing the strip and turns in a cylindrical containerhaving an axial shaft, and mounting a contact to the shaft and inslidable abutment with the exposed turns at the edge of the strip.

3. The method of manufacturing a resistor comprising the elements ofclaim 1 wherein the plating bath contains gold cyanide, and theelectrically conducting member placed in contact with the band of turnson the edge of the strip comprises electrically conducting wool.

References Cited in the tile of this patent UNITED STATES PATENTS1,773,135 Flanzer Aug. 19, 1930 1,939,902 Kaul Dec. 19, 1933 1,987,118Lodge Ian. 8, 1935 2,005,456 Creager June 18, 1935 2,023,603 Lodge Dec.10, 1935 2,058,525 Takanashi Oct. 27, 1936 2,134,870 Fruth Nov. 1, 19382,204,623 Ruben June 18, 1940 2,407,251 Christensen Sept. 10, 19462,688,681 Nyyssonen Sept. 7, 1954 2,744,859 Rines May 8, 1956

